Known filling level measuring devices comprise an aerial for transmitting and receiving radar waves or microwaves, and an electronics unit which generates the signals to be emitted and which evaluates the received signals.
Furthermore, potential separation may be provided. For considerations relating to measuring technology and for safety reasons the potential of the electrical supply line of a radar sensor should be separated from the potential of the, often metallic, filling level container. This may for example result in a reduction in noise effects during measuring, and in a reduction in the susceptibility to interference of the measuring signals. Furthermore, such isolation may result in increased safety, for example in relation to avoiding fires that may occur as a result of a short circuit or a defect in the electrical supply line or in the electronics of the filling level radar. Thus, undesirable spark-over could, for example, lead to ignition of, or damage to, the feed material.
Such potential separation may be provided in the microwave path. However, this may result in signal impairment of the microwave transmission signal and receiving signal in the form of signal attenuation or reflections due to mismatching. In particular in the case of very high microwave frequencies implementation may be made difficult because discrete components for separation may not be available, and line elements, as a result of their small dimensions, may only be produced with reproducibly good characteristics at great expense.
U.S. Pat. No. 6,956,382 discloses potential separation which is arranged outside the electronics unit, between the two-conductor (i.e. two-wire) loop and the output circuit. Regulating the loop current of the output circuit is separated from the two-conductor loop by means of potential separation.
Since in the case of sensors with a 4 . . . 20 mA two-conductor connection the current on the two-conductor loop may have to be regulated by the output circuit while at the same time potential separation between the two-conductor loop and the output circuit may have to be observed, it may be necessary to also transmit such signals across the non-conducting separation point, which signals may be less well suited to transmission across such a separation point. This relates not only to the current-sensing signal (the actual value of regulation), but also to the regulating signal of the regulator. Both signals are usually analogue direct-voltage signals; in the case of the current-sensing signal, moreover, exact transmission of the amplitude value may be necessary. This may not be possible with simple optical couplers. The expenditure for potential separation between the output circuit and the two-conductor loop may therefore be relatively high.